This invention relates to the hot water process for extracting bitumen from tar sands and, more particularly, to an improved conditioning or mulling drum in which the tar sands are submitted to a critical step in the process.
Tar sands (which are also known as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous, petroleum. Tar sands are found throughout the world, often in the same geographical areas as conventional petroleum. The largest deposit, and the only one of present commercial importance, is in the Athabasca region in the northeast of the province of Alberta, Canada. This deposit is believed to contain perhaps 700 billion-one trillion barrels of bitumen. For comparison, 700 billion barrels is just about equal to the world-wide reserves of conventional oil, 60% of which is found in the Middle East. While much of the Athabasca deposit is not economically recoverable on a commercial scale with current technology, nonetheless, a substantial portion is situated at, or very near, the surface where it may fairly readily be mined and processed into synthetic crude oil, and this procedure is being carried out commercially on a very large scale by Great Canadian Oil Sands (now Suncor Inc.- Oil Sands Division) and Syncrude near Fort McMurray, Alberta.
Athabasca tar sands are a three-component mixture of bitumen, mineral and water. Bitumen is the valuable component for the extraction of which tar sands are mined and processed. The bitumen content is variable, averaging 12 wt% of the deposit, but ranging from 0 to 18 wt%. Water typically runs 3 to 6 wt% of the mixture, and generally increases as the bitumen content decreases. The mineral content is realtively constant, ranging from 84 to 84 wt%.
While several basic extraction methods to separate the bitumen from the sands have been known for many years, the "hot water" process is the only one of present commercial significance and is employed by both Suncor and Syncrude. The hot water process for achieving primary extraction of bitumen from tar sands consists of three major process steps (a fourth step, final extraction, is used to clean up the recovered bitumen from downstream processing). In the first step, called conditioning, tar sands are mixed with water and heated with open steam to form a pulp of 70 to 85 wt% solids. Sodium hydroxide or other reagents are added as required to maintain pH in the range of 8.0-8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidliy floats (settles upwardly) to form a coherent mass known a froth which is recovered by skimming the settling vessel. A third stream, called the middlings drag stream, may be withdrawn from the settling vessel and subjected to a third processing step, scavaging, to provide incremental recovery of suspended bitumen.
Final extraction or froth clean-up is typically accomplished by centrifugation. Froth from primary extraction is diluted with naphtha, and the diluted froth is then subjected two a two-stage centrifugation. This process yields an essentially pure diluted bitumen oil product. Water and mineral removed from the froth during this step constitutes an additional tailings stream which must be disposed of.
As previously discussed, it is necessary to best operation of the hot water process that the tar sands be intimately contacted with steam and water in the initial mulling stage and that adequate agitation be applied to the mixture of tar sands and water to produce a pulp with a fairly uniform distribution of water. Proper contact of the tar sands with steam and water and proper mulling of the pulp is essential so that initial displacement of the sand particles from the bitumen can take place through the relative preferential affinity of the sand particles for water.
The physics of the separation of the bitumen requires that, in order to float, the bitumen be free from most of the mineral and contain enough gas to make the particles less dense than water. Also, the particles must be larger than about 30 microns in diameter in order to float in the time allowed. One observable effect of increased clay in the tar sands is to make the particles of oil smaller. When the sands are not conditioned properly, these flecks remain in the water-clay layer in the separation cell.
The previously preferred prior art conditioning drums are disclosed in Canadian Pat. No. 918,588, entitled "Hot Water Process Conditioning Drum", issued Jan. 9, 1973, to Marshall R. Smith, Frederick W. Camp, George H. Evans, and Jack E. Tinkler. Drums of the configuration disclosed therein have been employed for a number of years at the Suncor and Syncrude facilities. While this prior art conditioning drum has proved serviceable, experience at Suncor has revealed certain important drawbacks. It may be noted that this prior art conditioning drum emploiys a sparge valve as a distributing device which limits the discharge of the steam to those spider pipes instantaneously below the pulp level as the drum rotates. This concept was based on the theory that the heat transfer from the steam to the pulp is best achieved by injecting and condensing it directly therein. It has been found, however, that there are deficiencies in this original theory. The remaining spider pipes have been kept empty; therefore, the pipes and other components inside the drum and the drum shell itself have been exposed directly to ambient temperature. Thus, a majority of the spider pipes inside of the drum and the drum's shell and other internal components all remain cold and function as a cooling surface such that converse heat transfer takes place from the hot pulp to the drum structure. Because the majority of the drum structure remains much cooler than the pulp, heat transfer by radiation and convective heat exchange from the drum structure to the pulp is impossible. Further, the spider pipes (straight pipes with longitudinally arrayed rows of nozzles) distribute the steam along the drum in such a way that the majority of the steam under higher temperature is injected near the discharge end of the drum where the temperature of the pulp is the highest. Therefore, the conditions of steam condensation are poorest, and residence time of the pulp under high temperature inside of the drum is short.
Additionally, as a practical matter, there are very definite deficiencies in the prior art conditioning drum from the maintenance point of view. As previously noted, the steam is only sparged below the pulp surface which requires the use of a very large sparge valve as a mechanical distributing device. The sparge valve is a complicated and uncontrolled mechanical device inside of the drum. It is subjected to very rough service (sand, dirt, and even small rocks, pass through the sparge valve hub), and, as a result, lifetime of the sparge valve is unacceptably short and cannot be improved. In a related aspect, the prior art conditioning drums have only about 25% of the some 1200 nozzles under the pulp surface at a given time, but all of the nozzles have to be maintained in good condition and replaced by new nozzles continuously. It wil be apparent that a large stock of spare parts (bearings, seals, sparge valves, subassemblies, nozzles, etc.) must be maintained. Considering labor and time required for a regular sparge valve and nozzle change-out, this operation is manifestly very costly and causes production losses during the necessarily long drum shutdown.
From the foregong, it will be readily apparent to those skilled in the art that it would be highly desirable to provide a conditioning drum which is thermodynamically superior and simpler and less costly to fabricate and maintain than the prior art conditioning drum.